The present disclosure relates to motor control systems and, more particularly, to permanent split capacitor (“PSC”) motor control systems for use in heating, ventilation, and air conditioning (“HVAC”) applications.
Conventional HVAC applications often utilize multi-tapped PSC type motors. In general, a multi-tapped PSC motor is a motor that has a multi-tapped main winding where all or part of the main winding is coupled in parallel with an auxiliary starting winding that is coupled in series with a capacitor. Such multi-tapped PSC motors are used in HVAC applications, such as furnace blower and air handler applications, because the multi-tapped winding can produce variable output torque and, therefore, variable output speed for the purpose of delivering different amounts of air flow for different applications. For example, one tap setting may be provided to provide a relatively low amount of air flow to provide for air circulation when there is no heating or cooling activity. Another tap setting could be provided to increase the air flow when cooling is desired. By using multiple taps, various operating states can be established for a tapped PSC motor, such as heating, cooling, and air. In general, each tap point on the multi-tapped PSC motor is coupled to an input line and relays are energized in response to control signals from, for example, a thermostat to provide energization to one of the tap points at any given time.
One characteristic of multi-tapped PSC motors when used with air blowers, such as a squirrel cage blower, is that the Speed vs. Torque curves for such systems are not constant, but have a generally “reverse C shape” wherein the torque will increase with speed up to a maximum point but, thereafter, as the speed increases the torque will begin to decrease. FIG. 1 generally illustrates the Speed vs. Torque characteristics for a conventional multi-tapped PSC motor for low, medium, medium high and high settings with each setting having its own Speed vs. Torque curve. As the figure illustrates, for each Speed vs. Torque curve, as speed increases the output torque will initially increase from a minimum value at or near zero speed to a maximum value and then decrease to near or zero torque at a maximum speed.
In addition to having non-linear Speed vs. Torque characteristics, the operation of conventional multi-tapped PSC motors can be significantly impacted by the static pressure of the environment in which the system is operating. This is reflected by FIGS. 1 and 2, where FIG. 1 was described above, and FIG. 2 illustrates Static Pressure (in inches of water) vs. Air flow (in cubic feet per minute (CFM)) for the various taps of a conventional multi-tapped PSC motor. Lines reflecting average, low and high static pressures are illustrated in FIGS. 1 and 2.
As will be appreciated from FIGS. 1 and 2, for a given tap setting, as the static pressure is increased above the average static pressure value, the speed of the motor will increase. This speed increase will, therefore, result in a decrease in the output torque of the blower and accordingly a decrease in the output airflow from the blower. The reverse may occur if the static pressure drops below the average value. Because of this influence of the static pressure on the output airflow, in most HVAC systems using a multi-tapped PSC motor, the operation of the system will vary (perhaps significantly) from day to day, month to month as the static pressure within which the system operates changes. Such variations provide for unstable and inconsistent operation which is undesirable.
The present disclosure describes several embodiments a motor control system for a PSC motor that are designed to address the described and other limiting characteristics to conventional systems to provide an improved motor control system.